1. Field of the Invention
The present invention relates to an optical fiber and a nonlinear optical fiber, an optical amplifier and wavelength converter using the same, and a method of making an optical fiber.
2. Related Background Art
In general, it has been known that various nonlinear optical phenomena such as stimulated Raman effect and four-wave mixing occur in a medium when light having a high intensity (high optical density) propagates through the medium. These nonlinear optical phenomena also occur when light is transmitted through an optical fiber. Such nonlinear optical phenomena in the optical fiber can be used for optical amplification, wavelength conversion, and the like (see International Publication WO99/10770).
The nonlinearity of an optical fiber is represented by the nonlinear coefficient xcex3 in the following expression:
xcex3=(2xcfx80/xcex)xc3x97(N2/Aeff)
where xcex is the wavelength of light, N2 is the nonlinear refractive index in the optical fiber at xcex, and Aeff is the effective area of the optical fiber at xcex. This expression indicates that the nonlinear coefficient xcex3 can be made greater if the concentration of GeO2 added into the core of the optical fiber is enhanced so as to raise the nonlinear refractive index N2 while the relative refractive index difference between the core and cladding is increased so as to reduce the effective area Aeff.
When the configurational condition mentioned above is employed so as to increase the nonlinear coefficient xcex3, however, the cutoff wavelength xcexc of the optical fiber may become longer. When the four-wave mixing occurring in the optical fiber is used for carrying out wavelength conversion, in particular, it is necessary that the wavelength of excitation light be located near the zero-dispersion wavelength of the optical fiber. In the above-mentioned configuration, by contrast, the cutoff wavelength xcexc becomes longer than the zero-dispersion wavelength, so that no single mode can be attained, whereby the efficiency of wavelength conversion decreases.
In recent years, in order to widen the wavelength band of signal light used in optical transmission systems, the use of not only the amplification band of EDFA usually employed as an optical amplifier, but also S band in which wavelength ranges from 1.45 to 1.53 xcexcm, which is located on the shorter wavelength side of the former band, has been under consideration. For the S band, the EDFA is very difficult to be used since its amplification band is outside thereof, where by few effective amplifiers exist. If a Raman amplifier is to be used with a highly nonlinear optical fiber, the cutoff wavelength xcexc becomes longer than the wavelength of excitation light, which is about 1.3 to 1.5 xcexcm, whereby the efficiency in Raman amplification decreases.
For overcoming the foregoing problems, it is an object of the present invention to provide an optical fiber or nonlinear optical fiber exhibiting a shorter cutoff wavelength while having a sufficient nonlinearity, an optical amplifier and wavelength converter using the same, and a method of making an optical fiber.
For achieving such an object, the optical fiber in accordance with the present invention is characterized in that (1) it comprises, at least, a core region having a maximum refractive index value of n1; a first cladding region, disposed at an outer periphery of the core region, having a minimum refractive index value of n2 (where n2 less than n1); and a second cladding region, disposed at an outer periphery of the first cladding region, having a maximum refractive index value of n3 (where n2 less than n3 less than n1); and that (2) it has, as characteristics with respect to light having a wavelength of 1.55 xcexcm, an effective area of 11 xcexcm2 or less, a cutoff wavelength xcexcc of at least 0.7 xcexcm but not exceeding 1.6 xcexcm at a fiber length of 2 m, and a nonlinear coefficient of at least 18/W/km.
This optical fiber does not use a single-cladding structure but a double-cladding structure in which first and second cladding regions are disposed at the outer periphery of the core region. As a consequence, the cutoff wavelength xcexc can sufficiently be shortened even when, in order to increase the nonlinear coefficient xcex3, the concentration of GeO2 added into the core is enhanced so as to raise the nonlinear refractive index, or the relative refractive index difference between the core and cladding is increased so as to reduce the effective area Aeff. Also, this configuration can make the dispersion slope negative.
Here, as for the cladding structure, one or more other cladding regions each having a predetermined refractive index value and a width may be formed between the above-mentioned first and second cladding regions.
The nonlinear optical fiber in accordance with the present invention is the above-mentioned optical fiber characterized in that it utilizes a nonlinear optical phenomenon exhibited when a predetermined wavelength of light is fed therein. When the high nonlinearity in the optical fiber is actively utilized, a nonlinear optical fiber, applicable to various purposes, having a favorable characteristic can be obtained.
The optical amplifier in accordance with the present invention comprises (a) the above-mentioned nonlinear optical fiber having a cutoff wavelength xcexc; and (b) an excitation light source for supplying excitation light having a predetermined wavelength xcexp (where xcexc less than xcexp) to the nonlinear optical fiber with respect to signal light having a wavelength xcexs fed into the nonlinear optical fiber; wherein (c) a nonlinear optical phenomenon exhibited in the nonlinear optical fiber is utilized for optically amplifying the signal light.
Thus configured optical amplifier is utilizable as a Raman amplifier using the stimulated Raman effect occurring in the nonlinear optical fiber. Also, thus configured nonlinear optical fiber can make the cutoff wavelength xcexc shorter than the wavelength xcexp of the excitation light (pumping light), whereby optical amplification can be carried out with a high efficiency in a single mode.
The wavelength converter in accordance with the present invention comprises (a) the above-mentioned nonlinear optical fiber having a cutoff wavelength xcexc; and (b) an excitation light source for supplying excitation light having a predetermined wavelength xcex (where xcexc less than xcexp) to the nonlinear optical fiber with respect to signal light having a wavelength xcexs (where xcexc less than xcexs) fed into the nonlinear optical fiber; wherein (c) a nonlinear optical phenomenon exhibited in the nonlinear optical fiber is utilized for converting the wavelength of the signal light so as to output converted light having a wavelength xcexsxe2x80x2 (where xcexc less than xcexsxe2x80x2).
Thus configured wavelength converter is utilizable as a wavelength converter using the four-wave mixing occurring in the nonlinear optical fiber. Also, thus configured nonlinear optical fiber can make the cutoff wavelength xcexc shorter than each of the wavelengths of signal light, converted light, and excitation light, whereby wavelength conversion can be carried out with a high efficiency in a single mode. Further, the signal light can keep a favorable transmission characteristic without being affected by mode dispersion.
The method of making an optical fiber in accordance with the present invention comprises (1) a first step of preparing a core glass rod to become a core region made of SiO2 doped with a predetermined amount of GeO2 by synthesizing glass by VAD or OVD method and extending thus synthesized glass so as to attain a predetermined outer diameter; (2) a second step of preparing a first cladding glass pipe to become a first cladding region made of SiO2 doped with a predetermined amount of F by synthesizing glass by VAD or OVD method and extending thus synthesized glass so as to attain a predetermined inner diameter and a predetermined outer diameter; (3) a third step of heating the first cladding glass pipe while causing a predetermined gas to flow on an inner face thereof and carrying out etching for smoothing the inner peripheral surface thereof; (4) a fourth step of inserting the core glass rod into the first cladding glass pipe, baking the core glass rod and first cladding glass pipe at a predetermined temperature of at least 1300xc2x0 C., and then integrating the core glass rod and first cladding glass pipe together upon heating so as to yield an intermediate glass rod; (5) a fifth step of adjusting the ratio between the respective outer diameters of the core region and first cladding region in the intermediate glass rod, and then forming a glass body to become a second cladding region on an outer periphery of the intermediate glass rod so as to prepare an optical fiber preform; and (6) a sixth step of drawing the optical fiber preform upon heating so as to prepare an optical fiber comprising, at least, the core region having a maximum refractive index value of n1; the first cladding region, disposed at an outer periphery of the core region, having a minimum refractive index value of n2 (where n2 less than n1); and the second cladding region, disposed at an outer periphery of the first cladding region, having a maximum refractive index value of n3 (where n2 less than n3 less than n1); (7) wherein the core glass rod and first cladding glass pipe are integrated upon heating in the fourth step under a condition where the heating temperature is not higher than 1800xc2x0 C., the outer peripheral surface of the core glass rod has a roughness of 5 xcexcm or less, the inner peripheral surface of the first cladding glass pipe has a roughness of 5 xcexcm or less, and the GeO2 concentration in an area having a thickness of 2 xcexcm or less from the outer peripheral surface of the core glass rod has a maximum value of 5 mol % or less; and (8) wherein the optical fiber prepared in the sixth step has, as characteristics with respect to light having a wavelength of 1.55 xcexcm, an effective area of 11 xcexcm2 or less, a cutoff wavelength xcexc of at least 0.7 xcexcm but not exceeding 1.6 xcexcm at a fiber length of 2 m, and a nonlinear coefficient of at least 18/W/km.
Such a method of making an optical fiber can prepare an optical fiber of a double-cladding structure having a high nonlinearity with such a favorable transmission characteristic that, for example, the transmission loss is lowered.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.